1. Field of the Invention
The present invention relates to a radiography system and a radiography method, wherein tomographic images of a subject are obtained through reconstruction of radiographic images which are acquired from the subject by projecting radiation at different projection angles.
2. Description Related to the Prior Art
Radiography systems have been widely spread, which are each equipped with a radiation source for projecting radiation (e.g. x-rays) to a subject and a radiographic image detector for detecting radiographic images of the subject by detecting radiation that has penetrated the subject and falls on individual pixels of the detectors, which are arranged in a two-dimensional array. The radiographic image detected by the radiographic image detector is a so-called projection image, wherein information on the subject across the depth of the subject, i.e. in the radiation penetrating direction is superposed. Therefore, from a projection image of a subject (e.g. a patient for diagnosis), it is difficult to perceive multiple diseased tissues or the like, which may exist at different depths in the subject, distinguishably from each other.
Meanwhile, radiography systems have been known, wherein the radiation source and the radiographic image detector are configured to be movable in position or variable in projection angle so as to image a subject at different projection angles and thus acquire a sequence of projection images, and the sequence of projection images are subjected to a reconstruction process to produce tomographic images of the subject sliced at desired depths (US 2008/0108895 A1 corresponding to JPA No. 2008-114064 and US 2010/0195789 A1 corresponding to JPA No. 2010-172527). The imaging method in these radiography systems is called tomosynthesis. In comparison with the projection image, the tomographic image acquired by the tomosynthesis is improved in resolution in the direction across the depth of the subject, and it is possible to observe a diseased tissue or the like that exists at a certain depth distinguishably from another diseased tissue or the like that exists at a different depth.
Radiographic image acquisition is usually carried out by a radiologist according to a request from a doctor. After the radiographic image acquisition, the radiologist will examine the acquired projection images or tomographic images before submitting the images to the doctor (or to an image server or the like that the doctor can access for reference), in order to determine whether the acquired images are good or not, that is, whether the quality of the acquired images is adequate for use in diagnosis (hereinafter this examination will be referred to as the image examination).
Likewise, the image examination is carried out in the tomosynthesis. As for a tomosynthesis image acquisition, a plurality of tomographic images produced with regard to different positions in a subject, for example sequentially from one side to the other side of the subject, are to be inspected such that the inspected images are not biased in relation to the positions in the subject, thereby to determine on the basis of blurs, noise level, etc. of the tomographic images whether the image acquisition is successful or failed. For accurate determination on the success or failure of the image acquisition, it is usual to carry out the image examination of all tomographic images after the production thereof.
However, for the tomosynthesis, it is necessary to produce several tens to more than one hundred tomographic images, and it takes a long time to complete production of such a great number of tomographic images. The time waiting for the production of tomographic images is one of causes that waste time taken for the image examination.
The tomosynthesis imaging system according to US 2008/0108895 A1 automatically determines the sweep angle of the radiation source and the radiation dose, etc. by designated desirable image characteristics (such as resolution and noise level). This method may be effective to save the preparation time required for acquiring a set of projection images which are used for production of tomographic images, but could not reduce the time taken for the image examination.
The tomosynthesis imaging system according to US 2010/0195789 A1 produces an incomplete tomographic image using part of a set of projection images, to enable the image examination based on the incomplete tomographic image. However, as being produced using a reduced number of projection images, the incomplete tomographic image is inferior in quality to a tomographic image that is produced using all of the series of projection images. Therefore, depending on the skills in imaging and examining the tomographic images, there may be cases where the radiologist cannot exactly determine whether the image acquisition is successful or not on the basis of the incomplete tomographic image. Even while an incomplete tomographic image is displayed faster than a complete tomographic image and thus the radiologist may feel released from the stress of waiting for the production of an individual tomographic image for examination, the radiologist ends up waiting for the completion of all tomographic images of a subject because it is necessary to examine a plurality of tomographic images that relate to unbiased positions of the subject. Accordingly, the feature of producing and displaying incomplete tomographic images could not reduce the total time required for the image examination.